To obtain fluids, such as oil and gas, from a subterranean reservoir wellbores are drilled from the surface into the reservoir. The most commonly applied method to drill a well uses a derrick or mast structure, in which a drill string is assembled and continuously extended into the wellbore as the drilling progresses. Drilling is performed by rotating a drill bit attached to the end of the drill string. During the drilling process pressurized drilling fluid (commonly known as “mud” or “drilling mud”) is pumped from the surface into the hollow drill string. The main functions of the mud are to clean cuttings out of the wellbore, to cool the drill bit and to apply stress and pressure to the surface of the wellbore. The mud is carefully designed to achieve these objectives, the main parameters being its chemical composition, its additives and its density.
When the end of a drilling section is reached, the drill string and bit are withdrawn and steel pipe is lowered into the hole, which usually provides a fairly close fit to the hole. When the casing is in place, cement may be pumped down the inside and up the annulus and allowed to set. After such a casing operation, a further section of well may be drilled with a smaller diameter than the cased section.
Generally, the pressure exerted by the drilling fluid is greater than the formation or pore pressure so as to prevent the entry of formation fluids into the wellbore during the drilling process. As a beneficial side effect, when drilling through permeable rock, a small amount of pressurized mud may enter into porous sections of the formation from the wellbore and as it flows across the porous sections the mud may leaving behind a layer of larger particles on the wellbore wall. This layer left on the wellbore wall is commonly referred to as filter or mud cake. The mud cake may act to prevent further fluid loss into the permeable rock, which can be harmful, damaging formation permeability and lubricating fractures.
The barrier provided by the mud cake can potentially increase the so-called “mud window”. The mud window is a pressure range in which the driller maintains the mud pressure. The mud pressure should be sufficiently high to prevent influx from the formation whilst being low enough to prevent a fracturing of the formation and lost circulation. A wider mud window has the advantage of effectively increasing the distance that can be drilled before the open wellbore requires a casing. With an increased distance between subsequent casing shoes or points, the drilling operation can be completed in a shorter time period and at reduced costs.
Considerable efforts have therefore been made to optimize the mud cake as a protective layer—mostly by adding suitable chemical compositions to the base drilling fluid—in order to increase the stability of the mud cake and the adjacent formation or to increase the capability of the mud cake layer to isolate the wellbore from the surrounding formation.
In a specific branch of drilling techniques, casing may itself be used as the drill string so that the well is simultaneously drilled and cased. This method is commonly referred to as “casing drilling”. Under certain circumstances, casing drilling has been shown to reduce the in-hole trouble time significantly below that obtained by conventional drilling, hence reducing overall drilling costs.
Casing drilling has been identified as a technology that may be capable of reducing or minimizing problems associated with conventional drilling, such as stuck pipe, lost circulation, well control, and failure to run casing. It has been shown that the incidence of wellbore stability, lost circulation, influx and drag while tripping out are significantly reduced when using casing drilling compared to conventional drilling methods.
It has also been suggested that casing drilling provides a wellbore that is more stable and less permeable than drilling with a conventional drill pipe and collars. Desirable attributes associated with casing drilling may be at least partly attributable to so-called “mud cake” or the effects of mud cake as the process of casing drilling may mechanically strengthen the wellbore by building and maintaining an impermeable layer of the mud cake on the wellbore. US 2005/0167159A (Bailey et al.), incorporated by reference herein, discloses a mud suitable for use when casing drilling, comprising a conditioning additive designed to increase the strength of the filter cake.
However, for conventional drilling and even for casing drilling despite the improvements associated with such drilling technique, significant problems are still present particularly when drilling through impermeable material, particularly shale and mudstone formations, or when a transition from permeable to impermeable material is encountered.